This invention relates to methods of determining static gel strength of a fluid.
Gel strength is a characteristic that represents the ability of a fluid to support particulate material mixed in the fluid. In the oil and gas industry, this is an important characteristic to know about mixtures such as cement slurries and fracturing fluids, for example, because these mixtures must be able to support their particulate material while the mixtures are placed in their respective wells. Static gel strength relates to the force required to shear an undisturbed gel. This is an important characteristic of a cement slurry used in an oil or gas well because it relates to the ability of the cement to prevent gas leaks between formations.
Consistometers and viscometers are known devices that have been used in the oil and gas industry to measure various fluid characteristics. Typically consistometers are used to measure dynamic gel strength development; however, some consistometers are equipped to measure static gel strength (see U.S. Pat. Nos. 4,622,846 to Moon, Jr. et al. and 4,648,264 to Freese et al.).
A consistometer of at least this latter type includes a container in which a sample of the fluid to be measured is placed. A paddle is disposed in the fluid in the container so that the paddle can be rotated relative to the fluid and the container. The drive shaft of an electric motor is coupled to the paddle to rotate it at a selected speed. A torque sensing device is used to sense the force with which the fluid resists the movement of the paddle.
In such a consistometer, static gel strength is measured by attempting to penetrate the fluid with the paddle rotated at a slow speed. The paddle is rotated slowly so that the undisturbed (i.e., static) fluid is sheared by the paddle. The force required to shear the fluid defines the static gel strength.
The method by which such a consistometer has been used to determine this force has been one of trial and error wherein the paddle has been continuously rotated in the fluid sample. The speed at which the paddle has been rotated has been critical to obtaining an accurate measurement of the static gel strength. On the one hand, if the speed is too fast, the fluid "breaks" and presents lower resistance to the paddle than the actual static gel strength so that a lower force than presented by the actual static gel strength is sensed. On the other hand, if the speed is too slow, the maximum force that needs to be measured just prior to breakthrough for an accurate static gel strength measurement to be obtained will never be reached. Thus, to find that speed at which the maximum force is measured takes trial and error and expert knowledge of the measurement technique and fluid.
In view of the difficulty in obtaining accurate static gel strength measurements using the foregoing method, there is the need for an improved method by which static gel strength can be more readily obtained without the need for running a static gel strength test at different speeds and without the need for an expert to perform or analyze the test.